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Punnett Square Calculator

Blake Boege
Written by Blake Boege · Founder, Calculator Answers

A Punnett square calculator is a biological genetics aid that predicts the probability of offspring genotypes and phenotypes from a cross. Specifically modeling a monohybrid cross, this tool maps the combination of alleles from two parents onto a visual two-by-two grid. It calculates the resulting genotypic ratios (such as one homozygous dominant to two heterozygous to one homozygous recessive) and phenotypic ratios based on Mendelian rules of dominance. Science students and educators use this calculator to visualize genetic inheritance and understand how recessive traits can appear in subsequent generations.

Create a 2x2 monohybrid cross Punnett square. Enter parent allele genotypes to map genetic inheritance probabilities, find genotype/phenotype ratios, and study Mendelian genetics.

See also: Square Root Calculator, Square Footage Calculator – Sq Ft for Any Room, and How Many Square Feet in an Acre.

Quick Answer

Generate a monohybrid cross Punnett square. Enter the genotypes of both parents to calculate offspring combinations, genotype ratios, and phenotype ratios.

Punnett Square Results

PHENOTYPE RATIO

3 Dominant (A_) : 1 Recessive (aa)

Genotypes: 1 AA : 2 Aa : 1 aa

Parent 1 GenotypeAa
Parent 2 GenotypeAa
Genotype Ratio1 AA : 2 Aa : 1 aa
Phenotype Ratio3 Dominant (A_) : 1 Recessive (aa)

Visual 2x2 Grid

Aa
AAAAa
aAaaa
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Examples

Aa x Aa heterozygous monohybrid cross

Genotype = 1 AA : 2 Aa : 1 aa · Phenotype = 3 Dominant : 1 Recessive (3:1 ratio)

AA x aa homozygous cross

Genotype = 4 Aa · Phenotype = 4 Dominant (all offspring express the dominant trait)

Aa x aa test cross

Genotype = 2 Aa : 2 aa · Phenotype = 2 Dominant : 2 Recessive (50% probability for each)

How it works

The calculator combines the alleles from Parent 1 (columns) and Parent 2 (rows) to fill a 2x2 grid representing the four possible fertilization outcomes. It then tallies the occurrences of each genotype and determines the corresponding phenotype based on dominant/recessive rules.

Mendel's Laws of Inheritance

  • Law of Segregation: Parents pass down one random allele to their offspring.
  • Law of Dominance: The dominant allele hides the recessive allele in heterozygous combinations.

How to Read Punnett Square Ratios

When crossing two heterozygous parents (Aa x Aa), the resulting Punnett square yields four combinations: AA, Aa, aA (which is also Aa), and aa. This creates two distinct ratios:

  • Genotypic Ratio: 1:2:1 (representing 25% homozygous dominant AA, 50% heterozygous Aa, and 25% homozygous recessive aa).
  • Phenotypic Ratio: 3:1 (representing 75% displaying the dominant trait and 25% displaying the recessive trait).

Gregor Mendel and Monohybrid Crosses

Gregor Mendel, an Austrian monk, discovered these principles in the 19th century by breeding pea plants. By tracking single traits like plant height (tall vs. short) and seed color (yellow vs. green), he discovered that offspring inherit discrete units of information (which we now call genes) from each parent. His mathematical approach to tracking ratios laid the foundation for modern genetics.

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Frequently asked questions

A Punnett square is a visual grid diagram used in biology to predict the probability of offspring inheriting particular genotypes and phenotypes from a cross between two parents. It was named after geneticist Reginald Punnett.

To make a 2x2 Punnett square for a single trait (monohybrid cross), write one parent's two alleles along the top edge of the grid and the other parent's two alleles along the left edge. Fill in each cell by combining the corresponding column and row alleles. The resulting four cells represent the potential genetic combinations for the offspring.

Genotype refers to the actual genetic makeup or allele combination of an organism (such as AA, Aa, or aa). Phenotype refers to the physical expression or observable trait of that genotype (such as brown eyes or blue eyes).

A dominant allele (written in uppercase, e.g., 'A') is expressed even if only one copy is present (e.g., AA or Aa). A recessive allele (written in lowercase, e.g., 'a') is only expressed physically if two copies are inherited (e.g., aa) in the absence of a dominant allele.

Homozygous means an organism has two identical alleles for a trait (either homozygous dominant 'AA' or homozygous recessive 'aa'). Heterozygous means the organism has two different alleles for a trait ('Aa').

A monohybrid cross is a genetic cross between two parents that differs in only one specific trait. The standard cross of two heterozygous parents (Aa x Aa) yields a characteristic 3:1 physical ratio and a 1:2:1 genetic ratio in the offspring.